Now John has several options. 1) To lie and say that it was his mum who was driving. - OR - 2) Tell the truth that he himself was driving when the accident happened, which would reveal that Julia was lying to her husband. - OR - 3) Refuse to answer.
4) Point him back at his wife's reply. I would tell him he should clarify it with her.
Gives her a chance to tell him the truth, after he perhaps, ideally, reassured her he would understand if she had been protective of her "cub" and stick to both, no matter what.
My father was quite good at reassuring me that he would not punish me, no matter what I did, if only I told him the truth, so he would be able to help me out better than based on a lie, which would come out eventually.
He kept his word, never punished me, but helped.
Stefan, will you share that later on how it worked out?